here is a very simple function that should do it:
function loginout_button_wpa89153($echo = true){
if (is_user_logged_in()){
$url = wp_logout_url(get_permalink());
$anchor = "Logout";
}else{
$url = wp_login_url(get_permalink());
$anchor = "Login";
}
$button = '<a href="'.$url.'" class="join-button">'.$anchor.'</a>';
if ($echo)
echo $button;
else
return $button;
}